Television systems have traditionally used an interlaced format for picture display. In this system, images are displayed on half of the available horizontal lines at a time. The system alternates between using the odd and even numbered lines for display, and therefore the vertical resolution of the system is reduced by half. Images are displayed at 50 Hz (PAL/SECAM) or 60 Hz (NTSC) with each image on half the lines being known as a field.
Progressive-scan systems aim to increase vertical resolution by displaying an entire frame (an image using all the available lines) for every field of input information. This type of display is particularly useful when a film is being shown since each image is static and can be displayed on all the lines of the display in a single field. The simplest method of allowing interlaced material to be displayed on a progressive-scan system is by weaving the two most recent temporally adjacent fields together to produce a complete frame. FIG. 1 shows how this takes place. Fields 101, 102 and 103 occur sequentially. Fields 101 and 103 contain information from odd-numbered lines and field 102 contains information from even numbered lines. Frame 104, which contains information from both odd and even numbered lines, is constructed by weaving together the information from field 101 and 102. Frame 105 is constructed by weaving together fields 102 and 103 in the same way.
Combining temporally spaced images in this way is the ideal solution for perfectly static pictures—however it can result in unpleasant visual artefacts known as combing if the images contain motion. For example, the sequence of fields in FIG. 1 would appear to the viewer as a smooth fade from a dark to light colour. However the woven frames would appear to the user as a fade with horizontal stripes across the screen, or as a ‘shimmering’ effect. Methods exist to reduce the visual impact of these artefacts by combining the most recent field with a reconstructed field, however such methods inevitably result in some loss of vertical resolution over woven fields.
Therefore to make the choice between weaving two existing fields together and using a reconstruction algorithm which uses both temporal and spatial interpolation it is important to detect motion between fields.
Systems exist to detect motion between fields by spotting combing patterns. Examples are found in U.S. Pat. Nos. 6,348,949, 4,982,280, 5,291,280, 6,014,182. Straightforward systems are limited in operation to finding simple magnitude differences between the pixels. They are somewhat simplistic and can be unreliable and sensitive to noise. More reliable solutions also exist but involve complex filtering and processing.
In addition many systems operate in only one dimension of colour space. Such simplifications make the system less sensitive to motion. For example if luminance differences are used, objects moving over differently coloured backgrounds with equal luminance are not detected. Examples of this situation can be found in sequences intended for young children, or animated logos, where primary colours are often used.